Pyrotechnic bridge detonating circuit with zener diode circuit controlling switching of scr



Dec. 28, 1965 B. KAPP ET Al. 3,225,695

PYROTECHNIC BRIDGE DETONATING CIRCUIT WITH ZENER DIODE CIRCUIT CONTROLLING SWITGHING OF SCR Filed Aug. 4, 1961 2 Sheets-Sheet 1 Dec. 28, 1965 B. KAPP ET AL 3,225,695

PYROTECHNIC BRIDGE DETONATING CIRCUIT WITH ZENER DIODE CIRCUIT CONTROLLING SWITCHING OF SCR Filed Aug. 4, 1961 3 Sheets-Sheet 2 United States Patent O 3,225,695 PYROTECHNIC BRIDGE DETONATING CIRCUIT WITH ZENER DIODE CIRCUIT CONTROLLING SWITCHING F SCR Benjamin Kapp, Los Angeles, and R. Carr Wilson, Redendo Beach, Calif., assignors to Space Recovery Systems, Inc., El Segundo, Calif., a corporation of Delaware Filed Aug. 4, 1961, Ser. No. 129,295 2 Claims. (Cl. 102-70.2)

The present invention relates t-o an improved switching system which is insensitive to high frequency radiation or other electrical noise, but which will respond to a predetermined direct current signal.

The invention is more particularly concerned with such an improved switching system which is eminently suited for pyro'technic initiation and which renders the ignition of the pyrotechnic device immune to spurious signals.

The use of pyrotechnic devices in airborne applications and in spaced vehicles has become more and more prevalent in recent years. For example, many space vehicles include a nose cone which often is subsequently released and returned to ear-th for recovery purposes. The release of the nose cone from the space vehicle is often effectuated by deploying a parachute by pyro-technic means. The pyrotechnic means usually comprises a suitable pyrotechnic squib which is ignited by means of an electrical switching circuit.

The release of the nose cone from a space vehicle represents but one use for the electrically ignited pyrotechnic devices. In each application, however, it is of paramount importance rthat the pyrotechnic device does not become ignited and tired due to the action of spurious electric or magnetic fields or signals on the electrical ignition system.

Pyrotechnic devices are used generally, for example, throughout the aircraft and missile field wherever a need for physical work arises. At the present time these devices represent the best known source of comp-act power.

These devices are used, for example, for ignition purposes in the launching of missiles, In thi-s application, for example, it is of paramount importance that the stray ields created, for example, upon the initiation of associated radar equipment do not produce an ignition prior to the actual time the missile is to be launched.

' Pyrotechnic devices are als-o in present-day widespread use in space vehicles for starting gyros at launch and to instigate and control various projects in flight, such as, separa-tion of the nose cone as mentioned above; jettison of stages and equipment; operation of tests, and so on.

vAs mentioned above, electrically actuated pyrotechnic devices, in general, have a tendency to respond to electromagnetic and electrostatic radiations; and it is an important object of the present invention to provide a switching system which will counteract such a tendency.

The present invention provides an improved switching system which is conceived and constructed to be insensitive to noise, or other spurious signals, lor t-o stray electromagnetic or electrostatic iields, and which responds only to a particular applied direct current signal t-o perform a desired switching action.

The above-mentioned characteristics of the improved switching system of Athe invention renders the switching system ideal for use in pyrotechnic ignition systems. However, it will become apparent as the description proceeds that the improved switching system of the invention finds utility whenever a switching action is desired which is immune to extraneous electrical or magnetic eifects.

It is, accordingly, an object of the present invention to provide an improved electrical switching system which is insensitive to spurious radiations.

A further object of the invention is to provide such an improved switching system which i-s particularly suited for the initiation of pyrotechnic devices, and which renders the pyrotechnic devices insensitive to spurious radiations and to other electrical noise.

Although mechanical relay systems could conceivably be constructed to fulfill the electrical criteria of the systern of the invention, such relay systems are subject to mechanical failure and to spurious electrical response in 'the presen-ce 'of mechanical shocks and vibrations. In addition, such relay systems would have a tendency to be relatively bulky and heavy.

The sys-tem of the present invention, as will be described, is immune to high accelerations, shocks, vibrations or acoustical loads, insofar as its electrical transfer characteristics are concerned. Moreover, the system of Ithe invention is not subject to mechanical failure-in the presence of such conditions.

An-other object of the invention, therefore, is to provide such an improved switching system which is composed of relatively few, rugged components and inherently simple associated circuitry, s-o as to be capable of being packaged as .a relatively small, lightweight unit; and one capable of withstanding high mechanical shocks and vibrat-ions.

Another object is to provide such an improved switching system which may be directly coupled to the pyrotechnic ignition circuit, and which is capable of providing an electrocstatic shield for itself and for the associated pyrotechnic device.

A -still further object is to provide such a system which may be packed as a re-usable unit, and which has provisions for adequate testing prior to use.

The features of the invention which are believed to be new are set forth in the attached claims. The invention itself, however, may best be understood by reference to the following description, when taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a circuit diagram of a switching circuit constructed in accordance with one embodiment of the invention and which contains an internal source of power to be switched by the circuit;

FIGURE 2 is a circuit diagram of ya switching system constructed in accordance with a modification of the invention and which responds to an external source of power to be switched by the system;

FIGURE 3 is an i-sometric view of the system of FIG- URE `1 or FIGURE 2 encased in a suitable housing and of an associated pyrotechnic device; and

FIGURE 4 shows the units of FIGURE 3 plugged together.

The circuit of FIGURE 1 includes va pair of input terminals 10 to which the tiring signal or voltage is introduced. The circuit is designed to be immune to strong electromagnetic and electrostatic fields and to the resulting induced signals or voltages which could otherwise cause spurious activation oi the pyrotechnic bridge circuit s-o as -to detonate the associated pyrotechnic device.

One of the terminals 10 is connected to a resistor 12 which may, for example, have a. resistance of 1 kilo-ohm. 'Ihe other terminal 10 is connected to a capacitor 14 and to a pair of resistors 16 and 18. The capacitor 14 and resistor 16 are connected to the resistor 12 and to a resistor 20.

The capacitor 14 may have a capacity, for example, of 25 microfarads; the resistor 16 may have a resistance of 800 ohms; the resistor 18 may have a resistance of 4.7 kilo-ohms; and the resistor 20 may have a resistance of 1 kilo-ohm.

The circuit constants set out above, and others to be listed herein, represent merely appropriate and typical values for a circuit constructed in accordance with the concepts of the invention. These values, of course, are not intended to limit the invention in any way.

The resistor is connected to the anode of a usual Zener diode 22. The cathode of the Zener diode 22 is connected to the gate electrode of a silicon controlled rectifier switch 24, and to a resistor 26. The resistor 26 is connected to the resistor 18, and the resistor is further connected to the cathode electrode of the silicon controlled rectifier switch 24 and to the negative terminal of a battery 28. The resistor 26 may, for example, have a resistance of 680 ohms. The silicon controlled rectifier switch 24 may, for example, be of the type presently designated 3A60A. The battery 28 may, for example, be a 6 volt battery.

The anode electrode of the silicon controlled rectifier switch 24 and the positive terminal of the battery 28 are connected through an appropriate connector to a usual pyrotechnic ignition bridge 32. This ignition bridge operates in known manner to respond to a current passed therethrough to ignite an associated pyrotechnic device.

The silicon controlled rectifier switch 24 functions as a solid state switch, and when it is rendered conductive, it in effect places the battery 28 across the pyrotechnic bridge 32, so that a firing current fiows through the bridge to detonate the associated pyrotechnic device. The silicon controlled rectifier switch 24, as noted above, is a known device and is available on the market. Normally, conduction between the cathode and anode, and vice versa, of the switch is cut off. However, the application of a suitable control signal to the gate electrode of the switch produces a conductive path through the switch between the cathode and the anode.

As noted above, the circuit of FIGURE 1 serves to reduce the noise and other spurious electrical inputs to the pyrotechnic ignition bridge 32. This means that the bridge 32 is caused to respond only to the actual firing current, so that the system is immune to the effects of stray radiations and noise.

The circuitry of FIGURE 1 may be incorporated into an appropriate housing, and the battery 28 may also be included in the housing, as will be described. This eliminates external wiring from the battery to the ignition bridge 32, and it eliminates the attendant susceptibility of such external wiring to stray magnetic and radio frequency electrical fields. The direct inclusion of the ignition battery 28 in the housing also permits close coupling of the pyrotechnic device to the ignition battery 28 and to the silicon controlled rectifier switch 24.

The resistor 12 and capacitor 14 in the circuit of FIG- URE 1 form a low-pass filter, which serves to by-pass and attenuate strong high frequency noise signals. This circuit may also be used as a timing circuit to provide a desired time delay between the application of the actual direct current firing signal to the circuit and the activation of the silicon controlled rectifier switch 24 to energize the bridge circuit 32 and detonate the associated pyrotechnic device.

The spurious signal voltages which tend to be developed across the capacitor 14 are discharged through the resistor 16. It is evident that the time constant of the circuit formed by the capacitor 14 and resistor 16 must be less than that formed by the capacitor 14 and resistor 12. The value of the resistor 12 must, therefore, be greater than the value of the resistor 16.

The silicon controlled rectifier switch 24 will be rendered conductive only when the voltage across the resistor 16 exceeds a particular threshold. The circuit is constructed so that this threshold voltage is exceeded only when a direct current firing voltage of a particular value, for example, of 24 volts, is applied across the input terminals 10. The resistors 12 and 16 also serve as a voltage divider for the firing voltage so that the system will be insensitive to low amplitude signals.

The resistors 18, 20 and 26, and the Zener diode 22, serve as a decoupling circuit for preventing the spurious signals from reaching the silicon controlled rectifier switch 24, and for permitting only the actual direct current firing signal to the switch. The Zener diode 22 serves to block all voltages less than its specified reverse breakdown voltage.

Only in the presence of the actual direct current firing signal or voltage does the Zener diode 22 become conductive so as to establish a voltage across the resistor 26. This voltage, for example, may be of the order of 0.55 volt.

The silicon controlled rectifier switch 24 is a three junction semiconductor device with Thyratron-like characteristics. As indicated above, these switches are well known and available on the market. The switch 24, as explained above, will block both in its forward and reverse directions until it is rendered conductive by the application of a positive voltage to its gate electrode. Present-day silicon controlled rectifier switches, such as the switch 24, are capable of passing currents of the order of 5 amperes, for example, 50 milliseconds at 100 Fahrenheit ambient conditions.

As noted above, there are advantages in including the battery 28 directly in the casing which houses the switching circuit. However, in some instances, external arming may be desirable. For this latter purpose, the circuit of FIGURE l can be modified in the manner shown in FIGURE 2.

The circuit of FIGURE 2 is generally similar to the circuit of FIGURE l and like components have been designated by the same numerals. In the latter embodiment, however, the battery 28 is replaced by a capacitor 40 and a resistor 42. The external arming voltage is then applied across the input terminals 44. It is evident that the circuit is fully inactive until an appropriate arming voltage is applied to the terminals 44. The circuit is armed by the application of a direct current arming voltage or of an arming pulse of sufficient duration to charge the capacitor 40 to a predetermined value within a time determined by the time constant of the circuit formed by the capacitor 40 and resistor 42.

The value of the resistor 42 is chosen to limit the current fiow into the capacitor 40 to a predetermined value of, for example, 5 milliamps, or other appropriate value at the highest voltage expected or intended. The value of the capacitor 40 is such that the capacitor may store sufficient energy to ignite the associated pyrotechnic device when the silicon controlled rectifier switch is rendered conductive.

As mentioned, the resistor 42 also forms a time constant circuit with the capacitor 40. The circuit of FIG- URE 2, therefore, can be armed only in response to an arming signal having predetermined characteristics, t0 the exclusion of spurious signals; and, once armed, the circuit can be fired only in response to a particular firing signal, again to the exclusion of spurious signals.

The firing of the circuit of FIGURE 2 is similar t0 that of FIGURE l. When the silicon controlled rectifier switch 24 in the circuit of FIGURE 2 is rendered conductive in response to the proper firing signal, the capacitor 40 will discharge and igniting current through the silicon controlled rectier switch 24 and through the pyrotechnic bridge 32 to detonate the associated pyrotechnic device, but only in the presence of the proper arming signal.

As shown in FIGURES 3 and 4, the switching systems of FIGURES l or 2 can be included in a suitable housing 50. As shown, the housing can have a tubular configuration. By way of example, in a constructed ernbodiment, the tubular housing measured 5/s in diameter and 3%" in length. The tubular housing 50 is made of a suitable electrically conductive material to form an electrostatic shield for the enclosed system. In the constructed embodiment, the tubular housing was composed of steel, so that it would also serve as a magnetic shield.

A connector 52 is mounted at one end of the housing. This connector receives the terminals 10. The leads bearing the firing signal are plugged into the connector 52 by means of a suitable mating plug.

The connector 30 is mounted at the other end of the tubular housing 50, and this connector is adapted to be plugged into a mating connector 54 associated with a usual pyrotechnic device 56. The connector 54 is connected to the bridge circuit 32 described above.

As shown in FIGURE 4, for example, the casing of the connector 30 completely surrounds the connector 54 when the switch assembly is plugged into the pyrotechnic device. The casing of the switch assembly is, moreover, in electrical contact with the casing of the pyrotechnic device.

If so desired, further immunization, even in the presence of the most adverse high frequency conditions, can be realized by interposing ferrite high frequency attenuators in the input leads.

The arrangement shown in FIGURES 3 and 4 provides a complete electrostatic and electromagnetic shield for the switching system and for the pyrotechnic device. This not only serves to shield the switching system from electrostatic and electromagnetic radiations, but also shields the pyrotechnic material.

It has been found that the pyrotechnic material often deteriorates and loses its ignition ability, when such material is subjected to electromagnetic or electrostatic radiations.

The invention provides, therefore, an improved switching system for use, for example, in energizing pyrotechnic ignition bridges. The switching system of the invention is eminently suited for its intended use in that it exhibits a high degree of immunity to noise signals and to other stray signals and radiation fields.

While particular embodiments of the invention have been shown, modifications can be made, and it is intended in the claims to cover all such modications as fall within the scope of the invention.

We claim:

1. A system for applying a preselected detonating current from a source of detonating current forming an integral part of an independent detonating circuit that comprises a pyrotechnic detonating bridge circuit, said detonating circuit being actuated by a normally electrically isolated switching circuit which is -actuated by a ring voltage applied to said switching circuit from a source remote in space therefrom, comprising, in combination:

(A) an electrical switching circuit including,

(l) a low pass filter network having input and output terminals for passing without substantial attenuation at said output terminals all frequencies below a predetermined cut-oirr frequency appearing at said input terminals and blocking with substantially high attenuation at said output terminals all frequencies above said predetermined cut-off frequency, said low pass filter network including resistance and capacitance means connected in series across said input terminals with said capacitance means connected across said output terminals and a shunt resistor connected across said capacitance means and said output terminals:

(2) a Zener diode circuit coupled to said output terminals including first, second and third resistor means connected in series across said shunt resistor, and a Zener diode including an anode connected in series with said rst resistor means and a cathode connected in series with said second resistor means, said Zener diode having a high ratio of reverse to forward resistance until avalanche breakdown occurs at a predetermined avalanche breakdown voltage.

(3) means to apply a tiring voltage from 'said remote source of tiring voltage to said input terminals to produce said predetermined avalanche breakdown voltage across said Zener diode to drive said Zener diode into conduction for producing a switching voltage across said second resistor means proportional to said firing voltage, and

(4) a silicon controlled rectifier connected `across said second resistor means including an input electrode connected between said second and third resistors means, an output electrode and a gate electrode connected in series with said cathode, said silicon controlled rectier normally representing an open circuit to the flow of current and able to switch rapidly to a conducting state when said switching voltage is produced across said second resistor means; and

(B) a detonating circuit including a pyrotechnic detonating bridge circuit connected in series with a source of detonating current of predetermined value, said pyrotechnic bridge circuit connected in series with said output electrode and said source of detonating current connected in series with said input electrode so that said predetermined detonating current is applied to s-aid pyrotechnic bridge circuit only when said silicon controlled rectifier responds to said switching voltage.

2. The system of claim 1 wherein said source of predetermined detonating current is -a battery including a negative electrode connected in series with said input electrode and a positive electrode connected in series with said pyrotechnic detonating bridge circuit, said battery and said pyrotechnic bridge circuit being mounted in a unitary structure.

References Cited by the Examiner UNITED STATES PATENTS 2,906,206 9/1959 Morison et al l02-70.2 2,963,653 12/1960 Campbell 328-165 2,986,677 5/1961 Hechler 307-88.5 3,040,660 6/1962 Johnston 317-80 3,049,642 8/1962 Quinn 307--885 OTHER REFERENCES Electronic Design Zener Diode Characteristics, page 26, March 19, 1958.

ARTHUR GAUSS, Primary Examiner.

JOHN W. HUCKERT, Examiner. 

1. A SYSTEM FOR APPLYING A PRESELECTED DETONATING CURRENT FROM A SOURCE OF DETONATING CUREENT FORMING AN INTEGRAL PART OF AN INDEPENDENT DETONATING CIRCUIT THAT COMPRISES A PYROTECHNIC DETOANTING BRIDGE CIRCUIT, SAID DETONATING CIRCUIT BEING ACTUATED BY A NORMALLY ELECTRICALLY ISOLATED SWITCHING CIRCUIT WHICH IS ACTUATED BY A FIRING VOLTAGE APPLIED TO SAID SWITCHING CIRCUIT FROM A SOURCE REMOTE IN SPACE THEREFROM, COMPRISING, IN COMBINATION: (A) AN ELECTRICAL SWITCHING CIRCUIT INCLUDING, (1) A LOW PASS FILTER NETWORK HAVING INPUT AND OUTPUT TERMINALS FOR PASSING WITHOUT SUBSTANTIAL ATTENUATION AT SAID OUTPUT TERMINALS ALL FREQUENCIES BELOW A PREDETERMINED CUT-OFF FREQUENCY APPEARING AT SAID INPUT TERMINALS AND BLOCKING WITH SUBSTANTIALLY HIGH ATTENUATION AT SAID OUTPUT TERMINALS ALL FREQUENCIES ABOVE SAID PREDETERMINED CUT-OFF FREQUENCY, SAID LOW PASS FILTER NETWORK INCLUDING RESISTANCE AND CAPACITANCE MEANS CONNECTED IN SERIED ACROSS SAID INPUT TERMINALS WITH SAID CAPACITANCE MEANS CONNECTED ACROSS SAID OUTPTU TERMINALS AND A SHUNT RESISTOR CONNECTED ACROSS SAID CAPACITANCE MEANS AND SAID OUTPUT TERMINALS: (2) A ZENER DIODE CIRCUIT COUPLED TO SAID OUTPUT TERMINAL INCLUDING FIRST, SECOND AND THIRD RESISTOR MEANS CONNETED IN SERIES ACROSS SAID SHUNT RESISTOR, AND A ZENER DIODE INCLUDING AN ANODE CONNECTED IN SERIES WITH SID FIRST RESISTOR MEANS AND A CATHODE CONNECTED IN SERIES WITH SAID SECOND RESISTOR MEANS, SAID ZENER DIODE HAVING A HIGH RATIO OF REVERSE TO FORWARD RESISTANCE UNTIL AVALANCHE BREAKDOWN OCCURS AT A PREDETERMINES AVALANCHE BREAKDOWN VOLTAGE. (3) MEANS TO APPLY A FIRING VOLTAGE FROM SAID REMOTE SOURCE OF FIRING VOLTAGE FROM SAID TERMINALS TO PRODUCE SAID PREDETERMINED AVALANCHE BREAKDOWN VOLTAGE ACROSS SAID ZENER DIODE TO DRIVE SAID ZENER DIODE INTO CONDUCTION FOR PRODUCING A SWITCHING VOLTAGE ACROSS SAID SECOND RESISTOR MEANS PROPORTIONAL TO SAID FIRING VOLTAGE, AND (4) A SILICON CONTROLLED RECTIFIER CONNECTED ACROSS SAID SECOND RESISTOR MEANS INCLUDING AN INPUT THIRD RESISTORS MEANS, AN OUTPUT ELECTRODE AND THIRD RESISTORS MEANS, AN OUTPUT ELECTRODE AND A GATE ELECTRODE CONNECTED IN SERIES WITH SAID CATHODE, SAID SILICON CONTROLLED RECTIFIER NORMALLY REPRESENTING AN OPEN CIRCUIT TO THE FLOW OF CURRENT AND ABLE TO SWITCH RAPIDLY TO A CONDUCTING STATE WHEN SAID SWITCHING VOLTAGE IS PRODUCED ACROSS SAID SECOND RESISTOR MEANS; MEANS (B) A DETONATING CIRCUIT INCLUDING A PYROTECHNIC DETONATING BRIDGE CIRCUIT CONNECTED IN SERIES WITH A SOURCE OF DETONATING CURRENT OF PREDETERMINED VALUE, SAID PYROTECHNIC BRIDGE CIRCUIT CONNECTED IN SERIES WITH SAID OUTPUT ELECTRODE AND SAID SOURCE OF DETONATING CURRENT CONNECTED IN SERIES WITH SAID INPUT ELECTRODE SO THAT SAID PREDETERMINED DETONATIG CURRENT IS APPLIED TO SAID PYROTECHNIC BRIDGE CIRCUIT ONLY WHEN SAID SILICON CONTROLLED RECTIFIER RESPONDS TO SAID SWITCHING VOLTAGE. 